IMI is best appreciated in a cryptographic
setting [1]}: Intrinsic information flow between \(X\)  and \(Y\)  is
synonymous with information communicated via secret key
agreement [2]}, [3]} between \(X_t\)  and \(Y_{t+\tau }\)  while
\(Y_{t}\)  is an outside observer. Based on this ansatz, the amount of information
flowing intrinsically from \(X\)  to \(Y\)  is equal to the secret key agreement rate
\(S(X_{t};Y_{t+\tau }|Y_{t})\) . (Defined in Sec. REF .) Since
\(S(X_{t};Y_{t+\tau }|Y_{t})\)  is not computable in practice, intrinsic mutual
information \(\mathcal {I}_{X\rightarrow Y}\)  is used as a (workable) upper bound
on \(S(X_{t};Y_{t+\tau }|Y_{t})\)  to monitor information that flows intrinsically
from \(X\)  to \(Y\) .
